If that block of wood is your bridge, it may be quite a bit too small.

The strings spread out to almost twice the width of the nut on top, and my bridge is wider than that still.

In case you didn't know what to look for, I've included the Amazon listing for the bridge I used. Seven bucks ain't bad.

4 String Banjo Bridge New Banjo Parts Maple Ebony https://www.amazon.com/dp/B0196J71NO/ref=cm_sw_r_awdo_F8W4GC60HYKDVMG8T6M1